Eotragus lampangensis, Suraprasit & Chaimanee & Chavasseau & Jaeger, 2015

Suraprasit, Kantapon, Chaimanee, Yaowalak, Chavasseau, Olivier & Jaeger, Jean-Jacques, 2015, Middle Miocene bovids from Mae Moh Basin, Northern Thailand: The first record of the genus Eotragus from Southeast Asia, Acta Palaeontologica Polonica 60 (1), pp. 67-78 : 69-73

publication ID	https://doi.org/ 10.4202/app.2012.0061
DOI	https://doi.org/10.5281/zenodo.10989583
persistent identifier	https://treatment.plazi.org/id/03DD87B4-700D-C922-FF2B-F9042AA6FF70
treatment provided by	Felipe
scientific name	Eotragus lampangensis
status	sp. nov.

Treatment

Eotragus lampangensis sp. nov.

Figs. 2A View Fig , 3A–H View Fig , 4A View Fig ; Tables 2 View Table 2 , 3 View Table 3 .

Etymology: From the geographic location of the Mae Moh coal mine in the Lampang Province of Northern Thailand.

Holotype: UPP MM-68 , a left horn core with preserving part of the frontal bone.

Type locality: Mae Moh coal mine, Northern Thailand. Type horizon: Nakhaem Formation, coal layer Q, late middle Miocene (13.4–13.2 Ma) .

Referred material: From coal layer Q: UPP MM-69, right P2; UPP MM-70, right P3; UPP MM-57, left DP3; UPP MM-58 and UPP MM-71, left M3; UPP MM-59, right p2; UPP MM-60, left p2; UPP MM-61, right p4; UPP MM-62, fragmentary left mandible with p4 and m1; UPP MM-67, fragmentary right fragmentary mandible with m3; UPP MM-63 and UPP MM-66, right m1; UPP MM-64 and UPP MM-65, left m1; and from coal layer K: UPP MM-72, right p2; UPP MM-73, left m2; UPP MM-74, right m3; UPP MM-75, right astragalus.

Diagnosis.—Species of Eotragus similar in size to E. artenensis Ginsburg and Heintz, 1968 , with a long, slender, and scimitar-shaped horn core. The horn core is inserted slightly posterior to the orbit, and inclined posteriorly at about 45 o (relative to the frontal plane) and laterally at about 15 o (relative to the sagittal plane). The lateral outline of the horn core corresponds to an asymmetrical triangle. In cross section, the horn core is subcircular at its base, becoming elliptical and transversely compressed halfway along its length. A distinct anterior keel originates 1 cm above the base of thanterior face of the horn core, which also bears a faint posterior keel on its posterolateral surface. In lateral view, the anterior surface of the horn core is relatively concave, whereas the posterior surface is slightly convex or flattened.

Differential diagnosis.—Differs from most species of Eotragus , including E. clavatus , E. artenensis , E. noyei , and E. minus Ginsburg, Morales, and Soria, 2001 , in having a more vertically inserted and more laterally inclined horn core, as well as in the slender shape and comparatively great height of the horn core (compared to the size of its base) and the presence of a more distinct and sharper anterior keel; differs from E. artenensis in having a faint posterior keel of the horn core, more hypsodont tooth crowns, and relatively longer molars; differs from E. clavatus in its smaller size, as well as in having a faint posterior keel, more rugose tooth enamel, and a weaker lingual cingulum on M3; differs from E. halamagaiensis Ye, 1989 in its smaller size and less vertical insertion of the horn core, as well as the presence of a posterior keel and more distinct anterior keel.

Description.—A left horn core (UPP MM-68; Fig. 2A View Fig ) from the Q coal layer is completely preserved and attached to a portion of the frontal including the dorsal orbital rims. The preserved part of the frontal is thick and slightly elevated in the region of the inter-frontal suture. The long, slender, and scimitar-shaped horn core is inserted slightly posterior to the orbit, and inclined more posteriorly (45° relative to the frontal plane) and more laterally (about 15°relative to the sagittal plane) than in other species of Eotragus . In lateral view, the horn core is slightly curved anteriorly owing to its relatively concave anterior and slightly convex posterior surfaces ( Fig. 2A 2 View Fig , A 3 View Fig ), however, neither spiral nor anticlockwise torsion can be detected. In cross section, the horn core is subcircular at its base ( Fig. 2A View Fig 5), but turns elliptical (i.e., transversely compressed) halfway along its length, as indicated by the horn core compression index at 2 cm above the base ( Table 2 View Table 2 ). A distinct anterior keel originates 1 cm above the base of the horn core and runs along its anterior surface. The posterior side of the horn core is flatter than the anterior one, owing to the weak development of the posterior keel on its posterolateral surface ( Fig. 2A View Fig 4 View Fig ). The surface of the horn core becomes smooth near its base, with the surface of the pedicle being even smoother. The anterior portion of the pedicle is slightly higher than its posterior one.

All of the premolars and molars share a selenodont morphology with brachyodont crowns, although they are rather more hypsodont than those of cervids and tragulids from the same locality. The enamel is finely rugose. The upper molars lack lingual cingula. The anterior and labial cingulids of the lower molars are usually weak. In terms of the upper dentition, DP3 ( Fig. 3A View Fig ) has a triangular occlusal outline and relatively flattened lingual and posterior borders. The parastyle, mesostyle and metastyle are well developed as paracone and metacone ribs. The posterior lobe is more developed and much wider than the anterior one. Compared with the molars, the DP3 is relatively brachyodont.

P2 is characterized by a triangular occlusal outline and the lack of obvious cingula. This tooth is relatively wide posteriorly owing to a wide posterior style and a well-developed mesolabial crista and anterior style. The mesolabial crista is the highest cusp and situated close to the anterior style. P3 generally resembles P2 in its morphology, but is marked by a much wider posterior portion and more flattened posterior surface.

The flattened labial wall of M3 ( Fig. 3B View Fig ) appears oblique in occlusal view, and slightly inclined towards the lingual side. The postparacrista is attached to the premetacrista, corresponding to a straight labial wall in occlusal view ( Fig. 3B View Fig 1 View Fig ). The parastyle, paracone rib, and metastyle are less developed than the mesostyle. The protocone and paracone are situated higher than the metacone and metaconule. The anterior and posterior fossettes are rather narrow. Both the anterior and lingual cingula are either weak or absent. A small entostyle is present in UPP MM-71.

In the lower dentition, p2 ( Fig. 3C View Fig ), is distinctly smaller than p4. The highest part of p2 lies on the mesolabial conid. There is no anterior conid. By contrast, the anterior stylid, posterolingual conid, and posterior stylid are all well developed and extend to the lingual side of the tooth. The posterolingual conid is higher than the posterior stylid, but the latter does not fuse with the posterolingual cristid at the middle wear stage. In lingual view, the base of the crown is slightly concave ventral to the mesolingual conid ( Fig. 3C View Fig 2 View Fig ).

The p4 ( Fig. 3E View Fig ) is characterized by the presence of an anterior conid, a small posterolabial conid, an anterolingual cristid, and a posterolingual cristid, with the anterolingual cristid being more elevated than the posterolingual one. The anterior stylid is lower than the anterior conid. The posterolingual cristid is not attached to the posterior stylid. The posterior valley, which is smaller than the anterior valley, is open ( Fig. 3E View Fig 1 View Fig ).

The m1 ( Fig. 3D, E View Fig ) is smaller than the m2 and marked by a relatively smooth wall with two distinct metaconid ribs and a metastylid. The mesostylid, entostylid, and entoconid rib are weak. The metaconid is higher than the entoconid. The hypoconid is slightly lower and protrudes more labially than the well-developed protoconid. In lateral view, all of the major cuspids (protoconid, metaconid, entoconid, and hypoconid) are inclined anteriorly ( Fig. 3D View Fig 2 View Fig ). The internal postprotocristid does not fuse with the prehypocristid at the early wear stage. In the unworn UPP MM-66, the internal preprotocristid is fused with the pre-entocristid ( Fig. 3D View Fig 1 View Fig ). At the middle wear stage, the internal postmetacristid, internal postprotocristid, pre-entocristid, and prehypocristid are fused together and the preprotocristid joins the premetacristid ( Fig. 3E View Fig 1 View Fig ). The anterior and posterior fossettes are extremely narrow. The ectostylid is present labially, close to the prehypocristid. The anterior cingulum is weak.

The m2 (UPP MM-73; Fig. 3F View Fig ) closely resembles that of m1, but is distinctly larger ( Table 3 View Table 3 ).

The m3 is represented by an isolated and unworn specimen (UPP MM-74; Fig. 3I View Fig ), as well as a fragmentary mandible preserving a moderately worn m3 (UPP MM-67; Fig. 3H View Fig ). UPP MM-74 is rather elongated and bears a poorly developed hypoconulid. The ectostylid is present labially, but the posterior ectostylid is absent. By contrast, UPP MM-67 shows a well-developed hypoconulid and fusion of the posthypocristid and posthypoconulidcristid ( Fig. 3H View Fig ), as well as the well-developed ectostylid and posterior ectostylid. In addition, the anterior and posterior lobes are slightly more robust in UPP MM-67 than in UPP MM-74. Both specimens bear a weakly developed anterior cingulum and are characterized by a moderately flattened lingual wall with weak stylids and lingual ribs. The hypoconulid is low compared to other main conids. The metaconid is more elevated than the entoconid. The internal postprotocristid is attached to pre-entocristid without fusion. The prehypocristid is longer and more developed than the posthypocristid ( Fig. 3H View Fig ). The fossettes are extremely narrow and their labial faces are slightly steeper than their lingual ones.

The single and isolated right astragalus (UPP MM-75; Fig. 4A View Fig ) is characterized by a strong process on the proximal medial condyle, as well as an asymmetrical distal trochlea bearing a relatively shallow valley. The width of the distal trochlea is equal to its anteroposterior length. The stop facets of the posterior surface are relatively smooth. The astragalus of Eotragus lampangensis sp. nov. morphologically resembles that of E. clavatus from Sansan, France ( Fig. 4B View Fig ), E. artenensis from Corcoles, Guadalajara, Spain (Alférez et al. 1981), and E. minus from Dera Bugti, Pakistan, but is larger than the latter two and slightly smaller than in E. clavatus .

Geographic and stratigraphic range.—Mae Moh coal mine, Northern Thailand; Nakhaem Formation, coal layers of K and Q, late middle Miocene (13.4–13.2 Ma).